This invention relates to telecommunications switching systems and in particular to transmission gain control circuits in telephone switching systems.
Transmission gain control in telephone switching systems is usually performed in the trunk circuit under control of the marker. The system marker is provided information relating to the calling and called lines or trunks and inserts a predetermined pad into the transmission circuit to achieve gain equalization based on premeasured characteristics. However, in some telephone circuits, the predetermined value approach cannot be used since the transmission characteristics of the telephone circuit cannot be determined in advance. For example, remote access circuits have been developed which allow an ordinary subscriber to dial up a PBX from a remote central office. A special access trunk located at the PBX answers the call and sends a service request to the PBX as if the trunk appeared as a station at the PBX. The trunk subsequently cuts the central office line through to the PBX and the remote customer is then allowed to use PBX services such as tie lines and foreign exchange lines. If such a customer attempts to use a foreign exchange line, the resulting transmission quality may be so poor as to be unacceptable. Since the length of the foreign exchange line and its transmission qualities cannot be determined by the customer's serving central office, the predetermined pad equalization circuits in the central office trunk circuit cannot be used to compensate for the transmission loss caused by the foreign exchange line.
Various arrangements have been developed in the prior art which allow a customer to vary tranmission gain by means of amplifiers located at the station set. These arrangements, however, tend to be costly because an amplifier and associated controls must be provided at each station set.
Therefore, there appears to be a need for a customer controlled gain circuit which would allow the customer to adjust the transmission gain at the trunk circuit according to his present needs.
Accordingly, it is an object of the present invention to simplify customer controlled transmission gain circuits.
It is a further object of the present invention to eliminate station set amplifiers from customer controlled gain circuits.